I agree! weightlifting will strengthen your muscles to support your body which will eventually keep you from being injured. I also read this good article on how to avoid tennis injuries. I find this quite effective for me. Below is an explanation for the shoulder ache after playing tennis:

"The repetitive motion required by the serve and overhead can be stressful on the shoulder, which can result in soreness and can lead to a more serious injury such as a torn rotator cuff, says Dr. Plancher. Similarly the elbow and wrist are also at risk. Tennis elbow, which Dr. Plancher explains as an inflammation of the tendon that attaches to the ulna (top part of the elbow), one of the three bones that make up the elbow joint, is a common overuse injury among tennis players, particularly those over 30. Wrist strains are also common."

I agree! weightlifting will strengthen your muscles to support your body which will eventually keep you from being injured. I also read this good article on how to avoid tennis injuries. I find this quite effective for me. Below is an explanation for the shoulder ache after playing tennis:

"The repetitive motion required by the serve and overhead can be stressful on the shoulder, which can result in soreness and can lead to a more serious injury such as a torn rotator cuff, says Dr. Plancher. Similarly the elbow and wrist are also at risk. Tennis elbow, which Dr. Plancher explains as an inflammation of the tendon that attaches to the ulna (top part of the elbow), one of the three bones that make up the elbow joint, is a common overuse injury among tennis players, particularly those over 30. Wrist strains are also common."

You could maybe use tennis balls for under 10's for a little while. They bounce lower and are slower giving you more time to get into the right position to hit your shot. You could also do some exercises with your wrist and shoulder before and after playing your game.